Power supply VA rating and how much is really needed

VA = AC watts.

This is really the upper limit of what the power supply can deliver.

However, the actual wattage spent is the power consumed by the speaker divided by some efficiency factor. So
200 Watts at the speaker / .6 = 266 VA continuous required. Probably more. LOTS more if class A. Class D, use about 0.8

Also, watts at the speaker are impedance dependent. Half the impedance = 2x the power required. 
A stiff 200 Watt amp at 8 Ohms may do 400 Watts at 4. If very stiff, 800 Watts at 2 Ohms.

Still, the power supply limits all, so if you have 550 VA available per channel,  you'll be lucky to see 330 max at the speakers.
Further, this is all compounded by the fact that music is dynamic. Short term power delivery may be more important for music than steady state.

Fun stuff, right? :)
E

Got it.

Thx. I am not trying to drive in circles - but just for kicks --

So - question -- all things being equal in the audio world -- I know they are not, but given the following -- is one better than the other in dynamics, , sq simply because one has more power from the supply available? And will I sonically experience a difference in SQ? I know MP amps are not high end - but I am trying to wrap my head around specs in general.

1. monoprice ati clone 2ch -- Toroidal Transformers 1 (800VA)
2. monoprice ati clone 3ch -- Toroidal Transformers 1 (1,025VA)

I am only using the above amp as an example because MP happens to have a full spec sheet, unlike the others I am looking at.

Although I don’t see the spec that Odyssey, Rotel, or Parasound lists on their spec sheets which is Current Delivery.

So where does the current in ’amperage’ output delivery come into play here? As I understand it - more current output -- better dynamics, better sq... and from what I can gather - better amps -- have higher current output, which is one of many reasons they sound better.


Odyssey Stratos ( since it is one I am considering) lists the spec :

• 45 amps current delivery -- how does one calc this if it is not given on spec sheet? http://odysseyaudio.com/products-stratos-stereo.html

Not trying to be a dumbass - just trying to ’get it’.

In theory at least, the sound quality doesn’t degrade until you reach the limits of the amp.

Are you going to run into those limits? :)

I mean, the argument can be made that overkill is the best kind of kill when it comes to engineering, but there is more to the sound quality than merely capacity. Noise filtering, storage capacity, decoupling and signal routing of the power supply matter a great deal as well.
It’s not really a spec that a lot of people should use. Fortunately the one thing that’s pretty well defined in terms of measurements are how you rate an amplifier, which is in steady state after a warmup at 1/3rd power. So assuming an amp meets its specs it will deliver as promised and leave the details of the power supply to the engineers.
It is important to note the power, impedance and number of channels driven at that specification.

Again, not sure overkill is something you can hear. A well designed small amp that is never used outside of it’s design envelope could sound fantastic.

But... let’s say you have some troublesome speakers like an ESL, or one that dips to 2-3 ohms in the bass. That will require more current. An amplifier needs to have 3 things to not wilt on a difficult speaker:

1 - Adequate power supply (VA) for the output desired
2 - Appropriate driver and output stages
3 - Low output impedance.
So, imagine a perfect amplifier with no output impedance and a limitless power supply. No matter the speaker, the Voltage will not change.

On the other end are high output impedance amps. Typically tubes. Their output is a little dependent on the speaker’s impedance itself.
You can see this in Stereophile review of tube amps with the simulated speaker load.

Is that bad? Well, it’s not precise, but you might like how it sounds.

Best,
E

Kind of on this theme, Nelson Pass has written a great deal about how we test, measure and evaluate amps. He makes the convincing argument that the 1st watt is the most important one.

What the distortion or noise is at 250W is irrelevant because no one uses it. You want an amp that is low noise, low distortion and performs beautifully in the first few watts, and has power to not compress.

And one thing I find VERY true is that a lot of amp makers rate the S/N at full output. It tells you nothing about the relative noise. The most careful makers rate Signal to noise at 1 watt. That tells you whether a 50 W amp is noisier than a 500 W amp much more reliably.

@erik_squires 

Thanks I will read up on what Nelson Pass has written.

Based on what you have stated. And your knowledge base - 

given a budget of approx max $2k-$3k... 

all things being equal ... which do you think is a better amp based on specs? And which do you think would be a good fit for B&W speakers?

monoprice 1800/2007 ati clone?
odyssey stratos 2 or 3 ch or khartago version
rotel rmb-1585
emotiva xpa5 
outlaw 7700



I realize that how things sound is the end game -- and these are not top tier products - as they are all midfi at best... but I don't have the budget for the likes of Pass, Levinson, etc... 

Am I missing any contenders in the amplifier space that I should consider?